The present invention is directed to a method for determining the absolute pressure in the combustion chamber of an internal combustion engine with a pressure sensor.
The characteristic of a pressure sensor (the physical relationship between measured pressure and the sensor output signal) is kept within narrow tolerances for laboratory use by means of complex designs and/or calibration. The evaluation of the sensor signals occurs either with the characteristic defined by the manufacture or, for higher precision measurements, an additional calibration of the sensor is carried out immediately before a use.
In internal-combustion engines of motor vehicles, the absolute pressure in the combustion chamber is used e.g. for diagnostic and/or control of the combustion, for controlling knocking, for determining the work performed, etc.
In the automotive field, there is a need for inexpensive and reliable pressure sensors. Factors such as manufacturing tolerances, sensor aging, signal drift and temperature-dependency of the sensor make it difficult to produce an inexpensive and reliable pressure sensor as is needed in this automotive field. According to German published application 37 04 837, the maximum combustion pressure is calculated in a combustion-free condition (for example, deceleration) that extends over a plurality of work cycles and this is compared to a stored reference value. The maximum combustion chamber pressure identified during normal combustion is corrected using the difference or the ratio between these two values. A disadvantage of this method is that the correction value is identified in a first operating mode and is subsequently applied to signals that are determined in a different operating mode wherein entirely different combustion chamber conditions are present. Significant mismeasurements can arise as a result thereof.